In Doc's defense, KG did not look quite right even in the first quarter. He was still favoring his knee. Also, right before the Celtics started cutting into 16 point lead, Grande and Max mentioned that KG looked very uncomfortable out there before Doc took him out during the timeout.
IMHO, I though Doc made the right call. You never know with knee sprains and you're better off being safe than sorry. In the grand scheme of things, this game meant little compared to what could have happened.